There was a recall on some Winchester 7mm rem mag ammo a few years ago because of over pressure ammo. It is possible some of this ammo is still on dealers shelves. I'd try another type of ammo.

The belt on most magnum chamberings is mainly just for decoration as James K points out. It should headspace just like any other non belted case. It does add another factor that can cause issues for reloaders to have to work around.